1. Field of the Invention
The field of the present invention relates to a propeller for a boat.
2. Description of the Related Art
A variable-diameter type propeller for a boat is disclosed in U.S. Pat. No. 3,565,544, in which a propeller boss is fitted over and connected to a propeller shaft which is carried in a body of a propelling device to project rearwardly of the body, and a plurality of propeller blades are mounted to the propeller boss through a plurality of blade shafts disposed parallel to an axis of the propeller boss so as to surround the boss axis, so that the propeller blades can be turned between a closed position in which the propeller diameter is minimized, and an opened position in which the propeller diameter is maximized.
The exhaust system in an engine for a boat is classified broadly into two types: one being an underwater-exhaust system for releasing an exhaust gas into water through an exhaust passage extending through the propeller boss, and the other being an atmosphere exhaust system for releasing an exhaust gas into the atmosphere through an exhaust pipe arranged irrespective of the propeller. The underwater exhaust system is utilized mainly in an outboard engine, and the atmosphere exhaust system is utilized mainly in an inboard engine.
The prior art variable-diameter type propeller is used mainly in the inboard engine and hence, the underwater exhausting method is not taken into consideration at all.
A variable-thrust type propeller for a boat is disclosed, for example, in Japanese Patent Application Laid-open No. 144287/90, in which a propeller shaft supported in a body of a propelling device to project rearwardly of the body is connected with a propeller boss disposed rotatably about the propeller shaft through a torque limiting device which produces a slip, if it receives a torque equal to or more than a predetermined value, and a plurality of propeller blades are mounted to the propeller boss, such that the propeller diameter or the pitch angle can be varied.
In general, such variable-thrust type propeller includes the propeller blades and the torque limiting device arranged axially and hence, the axial length thereof is long as compared with a usual propeller having stationary blades. Thereupon, when such a conventional variable-thrust type propeller is employed, the propeller shaft carried in the body of the propelling device is formed as a long propeller shaft exclusively used for the variable-thrust type propeller. Therefore, the usual propeller cannot be mounted to the propeller shaft exclusively used for the variable-thrust type propeller without formation of an extra protrusion on the shaft. When the variable-thrust type propeller is replaced by the usual propeller, the propeller shaft must also be replaced by a short one used for the usual propeller. However, such a replacing operation is very troublesome, because of an attendant disassembling of the body of the propelling device.